1. Field of the Invention
The present invention relates to a method and to an apparatus for detecting biological activities in a specimen where the specimen and a culture medium are introduced into a sealable container and are exposed to conditions enabling metabolic processes to take place in the presence of microorganisms in the sample, the concentration of the initial substances being lowered and that of metabolic products being raised.
2. Description of the Prior Art
In many applications it is necessary to determine quickly whether a specimen is contaminated by microorganisms, such as bacteria, in particular in medical applications, in the pharmaceutical industry, food industry, or in environmental protection activities. The term "specimen" has a most comprehensive meaning here, including substances such as solid and liquid biological material (e.g. blood), food samples, such as frozen foods and preserves or canned foods, packaging material, clinical instruments and laboratory equipment, or samples taken from their surfaces, medical apparatus, first-aid and dressing material, soil and water samples, particularly samples of drinking water.
For a long time purely manual methods have been used in which the specimen to be assessed is placed in a culture bottle containing a liquid culture medium, and the growth of the culture is inspected only visually at given time intervals, and the type of presence of a microorganism is inferred from this observation by subculturing the liquid culture medium to a solid culture medium.
In addition, some technical procedures and devices are known, with which the biological activities in a sample are caused by microorganisms may be determined, and where the CO.sub.2 produced by the metabolism of the microorganism, or rather, the change in CO.sub.2 content, is employed as a measurement for determining the biological activity.
It is a known procedure, for example, to bottle the sample to be assessed together with a radioactively-labeled liquid culture medium and to test the atmosphere over the culture medium for radioactive gases, following which the presence of microorganisms in the sample may be determined.
Measuring systems of this type are described in U.S. Pat. Nos. 3,076,679 and 3,935,073, for example, fully incorporated herein by this reference. Although such systems are quick and reliable, they have certain disadvantages, i.e. radioactive substances must be handled and samples must be repeatedly taken from the gas space above the culture medium for frequent monitoring. When the samples are removed from the gas space, the remaining samples to be monitored may easily be contaminated by the sample-taking element and measuring errors may occur.
In the European application 0 158 497, a system is disclosed in which the biological activity of the specimen is determined by means of infrared absorption. In this method, a specimen is introduced into a sealable vessel containing a liquid culture medium, and is tested for the presence of microorganisms. The vessel is subjected to specific conditions, i.e. certain temperatures are maintained over given periods of time, thus enhancing the metabolism of the microorganisms, during which process CO.sub.2 is produced in the gas space above the culture medium by conversion of the carbon source. A sample is taken from the gas space and introduced into a measuring cell, and the CO.sub.2 content is measured by infrared absorption. Again, the subsequent samples may be contaminated, and another drawback is that infrared absorption is a less-sensitive means of measuring than radioactive labeling.
In order to avoid the problem of cross-contamination, the European application 0 104 463 proposes a method and a device which are also based on teh detection of CO.sub.2 (produced by metabolic processes) by means of infrared absorption. In this method, no sample is taken, but infrared radiation is directly transmitted through the wall of the vessel into the gas space above the culture medium, and its absorption is determined. Due to this non-invasive measuring method, cross-contaminations are largely eliminated; the disadvantage of this method, however, is its lack of sensitivity compared to radiometric methods; as well as the fact that the measurement is distorted by other gas components absorbing radiation in the same frequency band as CO.sub.2. A suitable example is the absorption bands of water vapor. The sample vessels employed must be transparent within a relatively narrow frequency range, which will only permit the use of specific materials for these vessels. An additional disadvantage is that the generation and filtering of the required infrared radiation is comparatively complex and expensive.
The European application 0 333 253 describes a device and apparatus for monitoring changes in pH and CO.sub.2 in a bacterial culture utilizing optical absorption measurements of a pH indicator in a matrix. Although it is not as sensitive as the radiometric method, it does offer the advantage of being noninvasive and can be continuously monitored. The primary disadvantage is that since color changes are being measured, different optical systems must then be used when the indicator medium is changed, thereby limiting the apparatus to one or two sensors.